Research in this section consists of studies on the physical and chemical properties of proteins of biological interest and the roles of ligand binding and protein-protein interactions in enzyme catalysis and regulation. (1) Interactions of divalent cations, substrates, and inhibitors with glutamine synthetase from E. coli have been studied by microcalorimetry, equilibrium dialyss, pH, spectral, and kinetic techniques. A reversible thermal transition in the enzyme structure is also being studied. In addition, labeling of the catalytic sites of the glutamine synthetase dodecamer has been achieved by reaction of this enzyme with pyridoxal 5'-phosphate, followed by borohydride reduction of the Schiff base. The pyridoxamine phosphate group acts as a spectral probe for the interactions of the modified enzyme with ligands. (2) Calorimetric studies with E. coli aspartate transcarbamoylase are in progress. (3) To produce a specific agglutinating activity, polymerization reactions of IgG antibodies are being studied.